When taking a photograph at high levels of ambient light, it is often desirable to use a source of artificial illumination, such as a flash bulb or electronic strobe flash unit, to provide supplementary light to the shaded portions of a photographic subject. When a source of artificial illumination is used in high ambient light, the camera and flash apparatus are operated in a mode commonly referred to as "fill-in flash". In fill-in flash applications, the supplementary light should lighten the shadows but not eliminate them. Proper film exposure thus requires apparatus that measures the amounts of flash light and ambient light and terminates the flash light and ambient light exposures when an appropriate ratio of ambient light to flash light has been provided.
U.S. Pat. No. 3,509,422 to Kilgore discloses a photo-flash lighting circuit which automatically provides an exposure having a constant ratio of ambient light to flash light. The circuit includes two photoelectric detectors for detecting and producing signals representative of the ambient light intensity and the reflected flash illumination, respectively, and a differential amplifier for detecting the difference between these signals to provide a signal that quenches the generation of light by the flashtube when a predetermined quantity of light energy from the flashtube has been reflected. However, for the best photographic effect, the ratio of ambient light to flash light should be changed for different scene contrasts. For example, in high contrast scenes having some portions brightly illuminated and other portions relatively dark, a three-to-one (ambient to flash) lighting ratio may be most appropriate where as in low ("flat") contrast scenes a six-to-one lighting ratio may be most appropriate.